1. Field of the Invention
The present invention relates to a control apparatus for a continuously variable transmission, and more particularly, to a control apparatus for controlling a lockup clutch and a forward clutch in the continuously variable transmission.
2. Discussion of the Background Art
In an automatic transmission having a torque converter with a lockup clutch, when the lockup clutch is engaged, an engine crankshaft is directly connected with an input shaft of the transmission and when the lockup clutch is disengaged, the direct connection is released, i.e., the torque converter is operative. When the lockup clutch is disengaged, the friction devices such as a low clutch, a high clutch and the like are engaged or disengaged to establish speed ratios of the transmission. When these friction devices are changed over from a released condition to an engaged condition and vice versa, the automatic transmission often causes so-called changeover shocks. Further, the automatic transmission sometimes causes engine over-runs or engine stalls due to a slippage of the changeover timing.
Japanese Patent Application Laid-open No. Toku-Kai-Hei 1-98758 discloses a technology in which the timing for temporarily releasing the lockup clutch is adjusted to the changeover time of the shift valve by a shift switch signal of the automatic transmission so as to eliminate the changeover shocks, engine over-runs or the like.
However, this technology is composed of timely combining two different operations, a shift operation of the automatic transmission and a temporary release of the lockup clutch. Accordingly, this control technology can not be applied to a case of continuously variable transmission where two control strategies, lockup control and clutch control, are performed by only one ON-OFF valve or switch valve.
It is an object of the present invention to provide a clutch control apparatus of a continuously variable transmission having a torque converter with a lockup clutch, by which changeover shocks that may occur when the lockup clutch is engaged or engine over-runs that may occur when the lockup clutch is disengaged can be prevented.
In order to achieve the object, the clutch control apparatus of the continuously variable transmission according to the present invention comprises a changeover means for changing over an oil passage connected to an apply chamber and a release chamber from an engaged position to a released position and vice versa, a slip pressure regulating means for regulating a slip pressure to be supplied to the release chamber, a lockup control judging means for judging whether or not a lockup clutch is to be engaged or to be released according to traveling conditions of the vehicle and a control means for reducing the slip pressure after a specified time elapses since the oil passage is changed over to the engaged position, when the lockup control judging means judges that the lockup clutch is to be changed over from the released position to the engaged position.